inlet to move the liquid into the pump. Atmospheric
pressure, acting on the surface of the liquid, then
provides the necessary pressure to move the liquid into
The principle of suction force, or suction lift, as
applied to reciprocating pumps, is shown in figure 6-6.
In diagram A, the piston cylinder is open at both the top
and bottom, so the liquid level at A and B is the same.
In diagram B, the cylinder is closed at the bottom. A
piston has been inserted and partly withdrawn, thus
creating a partial vacuum. When the foot valve (check
valve) at the bottom of the cylinder opens (diagram C),
as a result of the lower pressure in the cylinder, the
liquid rises up into the cylinder, which causes the
liquid level in the well to drop. Assuming the liquid is
water and there is a perfect vacuum below the piston,
atmospheric pressure pushes water up into the cylinder
to a height of 34 feet, even though the piston may be
raised higher than 34 feet.
You must understand that the preceding example
is for the theoretical condition of a perfect vacuum. In
practice, leakage between the piston and the cylinder,
friction (fluid) in piping, and gases dissolved in the
liquid limit the suction lift of a pump to a height of
approximately 22 feet, as shown in diagram D of
When a pump is pumping certain liquids, such as
hot water, oil, or gasoline, some of the liquid vaporizes
because of the vacuum on the suction side of the pump.
The pump may become vapor bound and reduce the
possible suction lift; this is called cavitation.
Figure 6-6.Diagrams showing the principles of
The suction force principle applies to other types
of pumps, as well as to the reciprocating type, though
to a lesser degree and in a different manner. The
centrifugal, the propeller, and the rotary pumps all use
suction force to a certain extent. Here, a partial vacuum
can be produced by the revolving mechanisms instead
of by the reciprocating plunger. Also, centrifugal
pumps are not self-priming because they do not pump
air. Their casing must be flooded before they can
function. In the eductor (jet pump), flow is maintained
by the suction force created by a jet of water,
compressed air, or steam passing through a nozzle at
high velocity. These principles are explained later in
VALVES USED WITH PUMPS
Every pump is equipped with devices for
controlling the direction of flow, the volume of flow,
and the operating pressure of the pump. A device that
performs one or more of these control functions is
called a VALVE.
A valve that permits liquid flow in only one
direction is classified as a CHECK VALVE. In most
cases, check valves open and close automatically; that
is, they are kept closed or seated by spring tension or by
the force of gravity until the liquid pressure above or
below the valve overcomes the spring or gravity
resistance and causes the valve to open. Check valves
of this type are used with centrifugal pumps to control
the suction and discharge of the liquid in the pump end
at the proper time automatically. Figure 6-7 shows a
vertical check valve. In this case, the valve is kept
seated by its own weight or the force of gravity. If
desired, it could also be kept closed by a spring.
Another type of valve in pump systems is the
STOP VALVE. Stop valves are usually opened or
Figure 6-7.Vertical check valve.